This invention is in the field of projection display systems, and is more specifically directed to the arrangement of optical elements in such a display system.
As is evident from a visit to a modern electronics store, the number of flat-panel (i.e., non-CRT) televisions has vastly increased in recent years, while the purchase price for such sets continues to fall. This tremendous competition is due in large part to the competing technologies for the display of high-definition television content. As known in the art, three major current display technologies for flat-panel televisions include liquid-crystal display (LCD), plasma display, and digital micromirror (DMD) based displays. The micromirror-based displays, and some LCD displays, are projection displays, in that a light source illuminates a spatial light modulator formed by the micromirror or LCD panel, with the modulated light then optically projected to a display screen. Plasma displays, on the other hand, are not projection displays; rather, each pixel at the display screen includes red, green, and blue phosphors that are individually excitable by way of argon, neon, and xenon gases, producing the image. Some LCD televisions involve “direct-view” displays, rather than projection displays, such that the liquid crystal elements are at the display screen and are directly energized to produce the image.
In modern micromirror-based projection displays, such as DLP® projection displays now popular in the marketplace using technology developed by Texas Instruments Incorporated, a digital micromirror device spatially modulates light from a light source according to the content to be displayed. An optical “engine”, which includes lens and mirror elements, projects the modulated light onto the display screen. As known in the industry, micromirror-based projection displays are advantageous from the standpoint of brightness, clarity, and color reproduction, as compared with other flat-panel televisions and displays. In addition, the use of micromirror spatial light modulators enable higher-speed modulation of light than many LCD systems, and micromirror-based systems have been observed to be extremely reliable over time.
However, conventional micromirror-based projection systems typically require larger “form factor” enclosures, than do LCD and plasma flat-panel systems of similar screen size and resolution. Two important measures of the enclosure for flat-panel display systems are referred to in the art as the “chin” dimension and the “depth” of the case. FIG. 1a illustrates the conventional definition of the “chin” of a flat-panel television, while FIG. 1b illustrates the “depth” of the system.
As shown in the front elevation view of FIG. 1a, display screen 2 is housed within enclosure 4. The portion of enclosure 4 that extends below screen 2 constitutes the “chin” of the display system. FIG. 1a illustrates dimension CHIN as the distance from the bottom edge of screen 2 to the bottom of enclosure 4. FIG. 1b illustrates, in connection with a side view of enclosure 4, the dimension DEPTH as the measurement between the front of enclosure 4 and the back of a rear-ward extending portion of enclosure 4. In micromirror-based projection display systems, the system components of the light source, digital micromirror, and the system projection optics, reside within the “chin” and the rearward extending portions of enclosure 4.
Consumers are attracted to televisions and display systems that are thin, from front to back, and for which the enclosure only minimally extends beyond the dimensions of the display screen itself. Indeed, it has been observed that the consumer buying decision is often based on the size of the enclosure for a given screen size. As mentioned above, the enclosures of modern plasma and direct-view LCD display systems can typically involve minimal chin and depth, because they are not rear projection systems and as such do not require enclosure of the light source, modulator, and projection optics required by projection systems, especially conventional micromirror-based systems. As such, these conventional micromirror-based projection systems are at a competitive disadvantage in the marketplace in this regard. And therefore, it is desirable for micromirror-based projection systems to also minimize the chin and depth of their enclosures, to attain and preserve market share.
In addition to the physical volume required for enclosures of projection display systems such as those based on micromirrors, other constraints also have resulted in substantial chin and depth dimensions. One such constraint is due to the TIR (Total Internal Reflection) Fresnel display screens that are now commonly used in projection display systems. As known in the art, TIR Fresnel display screens are capable of receiving light at a relatively steep angle from the normal, and of directing that light into the direction normal to the display screen, analogous to Fresnel lenses as used in traffic lights and lighthouses. This construction permits the source of the projected light to reside off-axis with the display screen, which greatly reduces the depth of projection display systems. FIG. 1c shows the rear projection of an image from source 8 (which may be a plane mirror, for example) to display screen 2, which is constructed as a conventional TIR Fresnel display screen. The angle of incidence of light from source 8 to the bottom of screen 2 is at a minimum angle φm from the normal, while the angle of light from source 8 to the top of screen 2 is at a maximum angle of incidence φx. It has been observed that, for conventional TIR Fresnel display screens, the minimum angle of incidence φm should be above 50° from the normal, to avoid flare and reduced contrast in portions of the displayed image. However, in order to achieve such a large minimum angle of incidence, it is therefore often necessary to construct an enclosure having substantial “chin”, as evident from FIG. 1c. In addition, if a plane mirror is used as source 8, to reflect the projected image to display screen 2, as shown in the conventional system of FIG. 1c, the minimum angle constraint commonly requires the height of this mirror to on the order of one-half the vertical dimension of display screen 2, especially as the depth of enclosure 4 is minimized.
Other design and manufacturing constraints also affect the design of conventional display “engines” for micromirror-based projection displays. These other constraints involve the nature of the light source (i.e., the “etendue” of the light), the extent of lens groups and numbers of lenses required to obtain a high resolution and minimum distortion image at the display.
By way of further background, a current trend in the construction of projection display systems is the use of non-telecentric lenses in the projection optics, between the spatial light modulator and the display screen. As known in the art, “non-telecentric” refers to lens arrangements that receive light from an image or source (i.e., the modulator) that is larger than the lenses; as such, the chief rays of light from various locations of the image are not parallel to, or not collimated with, one another. The use of non-telecentric lenses is popular in projection systems because the diameter of the lenses can be much smaller than the image or light source. Not only is the physical size of the lenses reduced, but the f-number of the lenses required for efficient light transfer is also kept relatively high, further reducing the cost of the lenses. As known in the art, large lenses of low f-numbers are relatively expensive to produce, especially for applications in which high image quality and resolution is important, as in high-definition television. It has also been observed that higher image contrast is generally attained by non-telecentric projection lenses. In addition, display systems using micro-mirror based spatial light modulators in combination with non-telecentric lenses can omit the “total internal reflection” (TIR) prism for separating “on” and “off” pixel light that is otherwise generally necessary with telecentric projection lens systems. Non-telecentric projection lenses are thus popular in modern projection display systems.
By way of further background, however, non-telecentric projection lenses are known to present certain limitations in projection display systems. Defocus caused by thermal or alignment effects at the SLM plane is made evident as dramatic magnification changes in the displayed image (“overfill” or “underfill”) in systems using non-telecentric projection lenses, even if the f-number of the projection lens group is relatively high (slow).
Another trend in the design of projection display systems is the use of wide-angle, high-magnification, aspheric mirrors as the element reflecting the projected image onto the display screen (e.g., as source 8 in the arrangement of FIG. 1c). It has been reported that the use of a high magnification aspheric mirror is believed to suppress color aberration.